When creating superflat worlds with certain presets unexpected behaviour results

Tried generating new worlds of some various superflat presets, seeing inconsistencies with spawn levels

Classic Flat - Player spawns on top layer at y=-60, as expected

Tunneler's Dream - Player spawns on top layer at y=173, as expected

Water World - Player spawns at y=~105, which is the spawn height in 1.16.5, and immediately falls 64 blocks. Expected to spawn at y=~41

Redstone Ready - Player spawns at y=56, the old spawn height, and falls 64 blocks. Does not deal fall damage on spawn (tested landing on dripstone and haybale as well). Expected to spawn at y=-7

NOTE: Tested several configurations of custom worlds, found that when the highest block at spawn point is grass block player seems to always spawn directly on ground level. This falls in line with the presets tested, as the ones which work correctly have grass as a top layer.

Able to reproduce all these consistently with respawning and new world generation

One potential fixes could be adjusting the presets to include more blocks of solid space, or adjusting the way spawn point is determined since it seems to currently rely on grass in some capacity